humping on male dogs is it pure dominance or just play sex

There are many dog behaviors that are normal, but can be contextually improper. Not just to people - to other dogs.

Dogs who know each other is a different situation than dogs who don't. Dogs, like people, will put up with more intimate or silly behavior from friends. If a friend hugs me, I'm happy. If a strange man does - not so happy.

From what I've seen at the dog park, dogs are similar. There are behaviors that are rude when they come from strangers. Humping tends to be one of them. Plus, ardent humpers usually ignore the body language that what is about to happen isn't welcome. That is like  a fool of a dog who tries to "play" by getting up in every other dog's face and annoying them.

Eko has no problem being humped by his friends. He doesn't like it when strange dog, or dogs he knows but clearly dislikes  try to hump him. He will sometimes tease male dogs whom he wants to play with by attempted humping (or nose bonking!). It's pretty obvious the point in those situations is "Hey come pay attention to me!"

I do not permit any humping that is unwelcome, or done with a one-up attitude. That doesn't tend to be his thing, but the few times he's tried it I stop him.

Willow has never tried this or has had it done to her.  But, if I ever saw this from her or another dog trying to do it to her I would immediately put a stop to it.  It's completely inappropriate behavior-IMO-and I've noticed some of the dogs I sit for become obsessed with it too. 

Canine Shakespeare:

To hump or not to hump. That is the question.

Whether 'tis nobler in the mind to suffer the slings and arrows of outrageous fortune

Or to take arms against a sea of troubles,

And by opposing end them? Ah, but to hump...

apologies to the Bard.

Both of my dogs get really PO'ed when another dog does that to them at the dog park. Like others have stated, I find it extremely rude and obnoxious - if someone's dog does that regularly they should not be at the dog park, period. I wouldn't take your dog back to the dog park until this behavior is stopped.